1. Field of the Invention
Nuclear reactors are frequently operated with uranium enriched in the isotope U.sup.235. The by-product of such enrichment is depleted uranium, commonly called "DU" in military circles. At typical enrichment levels, the production of a pound of enriched uranium will produce 125 pounds of depleted-uranium by-product, available as UF.sub.4.
All the U.S. military services are now using projectiles made from depleted-uranium alloys for armor penetration, and this usage is increasing rapidly. (No nuclear reactions are involved in such armor penetration.) Preparation of the projectile alloys involves the reduction of depleted-uranium UF.sub.4 with magnesium: EQU UF.sub.4 +2Mg=U+2MgF.sub.2 ( 1)
and the reaction may take place in stages: EQU 2UF.sub.4 +Mg=2UF.sub.3 +MgF.sub.2 ( 2) EQU 2UF.sub.3 +3Mg=2U+3MgF.sub.2 ( 3)
These reduction reactions are carried out in large, steel reaction vessels which are capable of withstanding transient internal pressures of hundreds of pounds per square inch when the vessel has been heated by an external furnace to a temperature of 700.degree. C. and when the reaction zone inside the vessel is at a transient temperature of 1600.degree. C. Magnesium metal powder in small excess and uranium fluoride powder are mixed and are placed in a reaction vessel with a liner to protect the steel against attack. Next, the vessel is sealed and heated to about 650.degree. C., at which temperature the magnesium-uranium fluoride charge ignites, heats itself to a high temperature, and forms molten magnesium fluoride and molten, depleted-uranium metal which separate as a fluoride layer floating on a metal layer. With successful reactions, frozen fluoride can be chipped off the metal.
As will be discussed below, the thermodynamics is satisfactory for the desired reactions. Specifically, if the system really could come to equilibrium following reaction of pure chemicals in unreactive vessel liners, the reaction products would be clean depleted uranium plus magnesium fluoride almost free of radioactive components. In fact, for true equilibrium conditions with high-purity chemicals in a pressurized vessel, the calculated content of uranium in the magnesium fluoride would be only about 10 parts per million. But that decontamination level is idealized.
In practice, the magnesium fluoride which is broken free from the frozen uranium contains a few percent of radioactive components as: (a) pockets of uranium oxide, (b) unreacted uranium fluorides trapped in the molten, magnesium-fluoride residue during the transient high-temperature reaction, (c) droplets of uranium which have failed to settle, e.g., those at a dirty metal-salt interface region, (d) unreacted materials from poorly sealed vessels, and (e) thorium decay products introduced from the uranium, especially Th.sup.234 with 24-day half life.
Substantially because of the radioactive components just mentioned, the magnesium fluoride residues from depleteduranium production cannot be discarded as simple chemical waste. Rather, they must be handled as mildly radioactive material, with all the associated expense, difficulties, and inconvenience of handling radioactivity. Specifically, without decontamination, disposal of radioactive magnesium fluoride costs about $100 per barrel in direct costs for shipment to burial and considerably more than that when indirect costs in factory inconvenience are included. No method or device has previously been developed to effect decontamination of the magnesium fluoride adequate to allow it to be treated as nonradioactive.
The present invention offers a new method of achieving the desirable decontamination through an unobvious combination of known chemical reactions which obey known thermodynamic relationships. Likewise, the present invention describes, but does not claim, apparatus uniquely designed to carry out the said decontamination.
2. Prior Art
Many publications tabulate thermodynamic enthalpies, entropies, and free energies for the formation of compounds of current interest, e.g., the Bureau of Mines Bulletin 605 and the JANAF Thermochemical Tables. By using techniques known to those versed in the art, the values tabulated can be modified to allow for new measurements published after the tabulations were prepared. Activity coefficients for components of solutions are known to be estimable from published phase diagrams and by analogy with similar systems for which better data are available. Such activity coefficients, when multiplied by component concentrations, give thermodynamic activities. Equilibrium relationships such as compositions of phases are known to be calculable from data such as these.
Although the chemical and thermodynamic information used for the present invention has been derived from publications in the open literature, the selection of suitable reactions and proper conditions from the multitude of possibilities is not obvious, even to one highly skilled in the art. Furthermore, the problems of magnesium fluoride disposal are recognized as troublesome by the military services, e.g., the Army Research and Development Center at Dover, New Jersey, whose personnel have been searching for a suitable decontamination procedure.
3. Definitions
The chemical reactions employed in the present invention, unless otherwise specified, may be applied for solutions as well as for pure components. To include both solutions and pure components, the term "magnesium reductant" is used to describe both metallic magnesium and magnesium alloys. Also, uranium fluoride, either alone or combined with other salts, may be reduced by magnesium reductant, thereby forming either uranium metal or uranium alloy; therefore, the term "uranium-fluoride" is used to describe both the pure material and its mixtures or solutions. Furthermore, the term "carbon" is used to describe the different elemental structures (such as amorphous carbon or graphite) as well as carbon compounds (such as silicon carbide).